Conventionally, CdHgTe is used as a material for an infrared detector, and, particularly, n-type CdHgTe is utilized as a material for a photo-conductive type infrared detector because it has a large photo-conductive effect that increases its conductivity due to the incidence of light. At present, an infrared detector that is obtained by epitaxially growing CdHgTe on a cadmium telluride substrate or on a cadmium zinc telluride (Cd.sub.x Zn.sub.1-x Te: x=0.93 to 0.95) substrate is used for a photo-conductive type infrared detector.
FIG. 15(a) is a cross-sectional view schematically illustrating a structure of a prior art photo-conductive type infrared detector, and FIG. 15(b) is a cross-sectional view illustrating a structure of a material substrate used for producing the photo-conductive type infrared detector. In the figures, reference numeral 201 designates a material substrate used for producing a photo-conductive type infrared detector (hereinafter referred to as "PC element material substrate"), and this PC element material substrate 201 comprises a CdTe substrate 3 and an n-type CdHgTe layer 11 epitaxially grown on the substrate 3. A PC-type infrared detector 201a is fabricated by using the PC element material substrate 201, Where a passivation film 22 and a ZnS reflection preventing film 23 are successively deposited on a light receiving region 201b at the surface of the n-type CdHgTe crystal 11, and Au electrodes 21 are produced at the both sides of the light receiving region 201b.
FIGS. 16(a) to 16(c) show a schematic construction of a liquid phase epitaxy apparatus and a growth method for growth of CdHgTe crystal. In the figures, reference numeral 4 designates a lower boat having a substrate mounting concave part 4a for mounting a crystal growth substrate, i.e., the CdTe substrate 3. An upper boat 5 including growth melt 8 and moving the same is slidably disposed on the lower boat 4, and a lid member 6 which can airtightly seal the upper boat 5 is attached to an upper aperture of the upper boat 5. The growth melt 8 comprises Te as solvent in which Cd and Hg as medium are dissolved in the proportion of 2 to 8. Reference numeral 10 designates an as-grown CdHgTe crystal, i.e., a CdHgTe crystal immediately after being grown on the CdTe substrate 3 by liquid phase epitaxy, that has not been annealed.
FIGS. 17(a) and 17(b) are diagrams for explaining a method of annealing the above described as-grown CdHgTe crystal 10. In the figures, reference numeral 31a designates an annealing furnace for annealing the as-grown CdHgTe crystal 10 that has a temperature distribution shown in FIG. 17(b). A quartz ampule 32 which will be thrown away after the usage is used for sealing the CdTe substrate 3 on which the as-grown CdHgTe crystal 10 is grown in a substrate containing part 32a and also sealing mercury 9 in a mercury containing part 32b. This quartz ampule 32 has a constricted part 32c for preventing the movement of the mercury at its one end side partitioning the mercury containing part 32b and the substrate containing part 32a.
A description is given of a prior art production method of an n-type Cd.sub.0.2 Hg.sub.0.8 Te crystal and a production method of the photo-conductive type infrared detector.
First of all, the CdTe substrate 3 is disposed in the substrate mounting concave part 4a of the lower boat 4, the upper boat 5 is positioned at one side of the lower boat 4, the growth melt 8 is put in the upper boat 5, and the lid member 6 is placed to airtightly seal the upper boat 5. Thereafter, the whole of the boat is heated to about 500.degree. C. in a hydrogen ambient, thereby melting the growth melt 8 in the upper boat 5 (FIG. 16(a)).
Subsequently, the whole is cooled down to a prescribed temperature, i.e., 480.degree. C. at a constant cooling rate, and the upper boat 5 is moved by sliding to a position above the CdTe substrate 3 from the side portion of the lower boat 4 as shown in FIG. 16(b). Thus, the melted growth melt 8 covers the surface of the CdTe substrate 3, and the CdHgTe crystal 10 is epitaxially grown on the CdTe substrate 3.
After a prescribed time passes, the upper boat 5 is moved by sliding to the other side of the lower boat 4 from the position above the CdTe substrate 3, thereby completing the crystal growth of CdHgTe (FIG. 16(c)).
Since the above described CdHgTe crystal includes mercury which has a quite high vapor pressure as a constitutional element, it is quite likely to generate mercury vacancies in the crystal, thereby easily deviating from stoichiometry, i.e., the ratio of Cd to Hg in the Te solvent (0.2:0.8), so that these mercury vacancies behave as acceptors in the CdHgTe crystal. Accordingly, the as-grown CdHgTe layer 10 grown as described above becomes a p-type semiconductor layer having a carrier concentration of about 10.sup.16 to 10.sup.17 cm.sup.-3 due to the mercury vacancies generated during its growth.
Because the p-type CdHgTe layer does not have so large a photo-conductive effect, this layer should be converted to n-type to be used as a material of a photo-conductive type infrared detector. Particularly, it is suggested in Material Research Society Symposium Proceeding Vol. 90 (1987) pp. 15, that the n-type carrier concentration of the material substrate should be below 10.sup.15 cm.sup.-3 to realize the photo-conductive type infrared detector.
From the above described reason, the as-grown CdHgTe crystal 10 obtained by liquid phase epitaxy is converted to an n-type CdHgTe crystal having a carrier concentration of 10.sup.14 cm.sup.-3 order by annealing. The annealing method of the as-grown CdHgTe crystal will be described in the following.
First of all, a CdTe substrate 3 on which the CdHgTe crystal 10 is grown by liquid phase epitaxy and mercury 9 are sealed in vacuum in the quartz ampule 32 as shown in FIG. 17(a), and this quartz ampule 32 is placed in the annealing furnace 31a having a temperature distribution as shown in FIG. 17(b), where the abscissa is positioned in the furnace in the longitudinal direction and the ordinate is temperature, thereby performing an annealing.
Then, the mercury partial pressure generated by evaporation of the mercury 9 is applied to the as-grown CdHgTe crystal 10, and the mercury atoms are diffused into the as-grown CdHgTe crystal 10. Therefore, the mercury vacancies in the crystal 10 are not only filled but the mercury atoms enter into the crystal as an n-type impurity, thereby converting the conductivity type of the as-grown CdHgTe crystal 10 from p-type to n-type. The annealing conditions are established so that the n-type carrier concentration of the CdHgTe crystal is set to a low value of 10.sup.14 cm.sup.-3 order. Thus, by a simple processing of only annealing the as-grown CdHgTe crystal 10 in the mercury ambient, an n-type Cd.sub.0.2 Hg.sub.0.8 Te crystal 11 appropriate for producing a photo-conductive type infrared detector is obtained.
Then, a passivation film 22 is produced by anodic oxidation of the light receiving region 201b of the n-type CdHgTe layer 11, and the ZnS reflection preventing film is produced, and thereafter the Au electrodes 21 are produced at both sides of the light receiving region 201b, thereby completing the photo-conductive type infrared detector 201 shown in FIG. 15(a).
A description of a photo-voltaic type infrared detector and a material for producing the same will be described in the following.
FIG. 18(a) is a cross-sectional view illustrating a structure of a prior art photo-voltaic type infrared detector, and FIG. 18(b) is a cross-sectional view illustrating a structure of a material substrate used for producing the photo-voltaic type infrared detector of FIG. 18(a). In the figures, reference numeral 202 designates a material substrate for producing a photo-voltaic type infrared detector (hereinafter referred to as "PV element material substrate"). This material substrate 202 comprises a CdTe substrate 3 and a p-type CdHgTe layer 12 epitaxially grown thereon.
Reference numeral 202a designates a PV-type infrared detector fabricated using the PV element material substrate 202, and this is usually used with a reverse bias of about 50 mV being applied. An n-type CdHgTe region 24 is produced at the surface of the p-type CdHgTe layer 12 of the PV-type infrared detector 202a by ion implantation, and a pn junction surface 24a is produced at the interface between the region 24 and the p-type CdHgTe layer 12. An insulating film 25, which also serves as an ion implantation mask, is produced on the surface of the p-type CdHgTe layer 12. An n-side electrode 26a having a two-layer structure comprising a lower Cr layer and an upper Au layer is formed on the n-type CdHgTe region 24. A p-side electrode 26b comprising Au is formed at a position spaced a prescribed distance apart from the n-type CdHgTe region 24 on the p-type CdHgTe layer 12.
FIGS. 19(a) and 19(b) show a method of annealing CdHgTe as disclosed in, for example, Japanese Published Patent Application Hei No. 1-148799. In the figures, reference numeral 31b designates an annealing furnace for annealing the as-grown CdHgTe crystal 10, having a temperature distribution as shown in FIG. 19(b). A quartz ampule 33 which will be thrown away after the usage is used for vacuum-sealing the CdTe substrate 3 on which the as-grown CdHgTe crystal 10 is produced.
A description is given of a prior art production method of the p-type CdHgTe crystal and a production method of the photo-voltaic type infrared detector.
First of all, similar to producing the n-type CdHgTe crystal, after producing the as-grown CdHgTe layer 10 by growing CdHgTe on the CdTe substrate 3 as shown in FIGS. 19(a) to 19(c), the CdTe substrate 3 is sealed in vacuum in the quartz ampule 33, and this quartz ampule 33 is contained in the annealing furnace 31b having a uniform temperature profile as shown in FIG. 19(b), where the abscissa is distance and the ordinate is temperature. Thereafter, an annealing of the as-grown CdHgTe crystal 10 is performed, whereby the p-type CdHgTe crystal 12 used for producing the photo-voltaic type infrared detector 202 is obtained.
Thereafter, after the insulating film 25 of a prescribed pattern is formed on the surface of the p-type CdHgTe crystal 12 obtained by the above described LPE method, ion implantation is carried out using boron or the like as an n-type impurity using the insulating film 25 as a mask, thereby selectively producing the n-type CdHgTe region 24. Thereafter, the n-side electrode 26a having a laminated structure of Cr/Au is formed on the n-type CdHgTe region 24, and the p-side electrode 26b comprising Au is formed at a portion in the vicinity of the n-type CdHgTe region 24 on the p-type CdHgTe crystal 12. Thereby, the photo-voltaic type infrared detector 202a shown in FIG. 18(a) is obtained.
In this photo-voltaic type infrared detector 202a, carriers which are generated in the p-type CdHgTe crystal 12 due to the incidence of infrared light are captured at the pn junction 24a, and they are taken out as a signal from the n-side electrode 26a, thereby detecting the infrared light.
Among the factors determining the performance of the above described infrared detector 202a, one of the most important is a dark current. As the dark current is lower, the signal to noise ratio (S per N ratio) of the infrared detector can be higher.
Generally, in an ideally produced infrared detector, a dark current is determined by the diffusion current generated by minority carriers which diffuse to the depletion layer and the generated and recombination current that is generated via the traps existing in the depletion layer. In the prior art photovoltaic type infrared detector 202a, the n-type CdHgTe region 24 is produced by ion implantation, and a region including defects generated by the ion implantation exists in the vicinity of the pn junction 24a, and therefore the influences of the generated and recombination current in this defective region on the characteristics of the infrared detector are large, thereby deteriorating the performance of the detector.
It is suggested in Journal of Vacuum Science and Technology A5(5), (1987) 3166, that due to the existence of n-type background impurities, i.e., impurities having a concentration not enough to determine the conductivity type in the p-type semiconductor layer, a pn junction can be produced at a deep position in the p-type semiconductor layer, i.e., at a position spaced from the surface, and a structure in which the conductivity type changes in the order of n-type, n.sup.- -type and p-type from the surface is realized. Therefore, the pn junction can be produced at a region spaced from the region that includes defects due to the ion implantation.
In other words, when ion implantation of an n-type impurity is carried out into an as-grown CdHgTe crystal including n-type background impurities, the mercury atoms in the vicinity of the surface are expelled from the lattice, and they may be present in the CdHgTe crystal as interstital mercury at a position deeper than the region where implanted elements are present.
By the annealing after ion implantation, the above described interstitial mercury atoms are diffused to the inside of the crystal so as to fill mercury vacancies. Therefore, the concentration of mercury vacancies at a deep portion in the CdHgTe crystal is decreased, and the conductivity type at this portion is controlled by the added background impurities and inverted to n.sup.- -type, thereby producing a pn junction at a position spaced from the surface of the CdHgTe crystal (refer to FIG. 11).
By employing the background impurity as described above, the pn junction 24a is not produced by ion implantation, but is produced by the diffusion of the interstitial mercury generated by the ion implantation, and therefore the pn junction 24a is produced at a deep region in the CdHgTe crystal where almost no defects generated by the ion implantation exist, thereby reducing the generated and recombination current by a large margin.
In the prior art production method of an n-type CdHgTe crystal, although the deviation from stoichiometry is controlled by the annealing of the undoped crystal in the mercury ambient thereby to control the n-type carrier concentration to about 10.sup.14 cm.sup.-3, it is difficult to completely control the stoichiometry because of the fluctuations in the furnace temperature and the variations in the degree of sealed vacuum, whereby the n-type carrier concentration largely varies from run-to-run, i.e., for each production of the n-type CdHgTe crystal, resulting in difficulty in producing an n-type CdHgTe crystal of a constant n-type carrier concentration at high yield.
In the prior art growth method of a p-type CdHgTe crystal, because there is no disclosure of the quantity of the background impurity to be added in the above-described reference, it is difficult to obtain a p-type CdHgTe crystal including background impurities and having a pn junction with a depth position that can be controlled stably.